subscribe-remote-state.md

# 远端状态订阅开发实例

IPC/RPC提供对远端Stub对象状态的订阅机制， 在远端Stub对象消亡时，可触发消亡通知告诉本地Proxy对象。这种状态通知订阅需要调用特定接口完成，当不再需要订阅时也需要调用特定接口取消。使用这种订阅机制的用户，需要实现消亡通知接口DeathRecipient并实现onRemoteDied方法清理资源。该方法会在远端Stub对象所在进程消亡或所在设备离开组网时被回调。值得注意的是，调用这些接口有一定的顺序。首先，需要Proxy订阅Stub消亡通知，若在订阅期间Stub状态正常，则在不再需要时取消订阅；若在订阅期间Stub所在进程退出或者所在设备退出组网，则会自动触发Proxy自定义的后续操作。

## 使用场景

这种订阅机制适用于本地Proxy对象需要感知远端Stub对象所在进程消亡，或所在设备离开组网的场景。当Proxy感知到Stub端消亡后，可适当清理本地资源。此外，RPC目前不提供匿名Stub对象的消亡通知，即只有向SAMgr注册过的服务才能被订阅消亡通知，IPC则支持匿名对象的消亡通知。


## Native侧接口

| 接口名 | 返回值类型 | 功能描述 |
| -------- | -------- | -------- |
| AddDeathRecipient(const sptr\<DeathRecipient> &recipient); | bool | 订阅远端Stub对象状态。 |
| RemoveDeathRecipient(const sptr\<DeathRecipient> &recipient); | bool | 取消订阅远端Stub对象状态。 |
| OnRemoteDied(const wptr\<IRemoteObject> &object); | void | 当远端Stub对象死亡时回调。 |

### 参考代码

```C++
#include "iremote_broker.h"
#include "iremote_stub.h"


//定义消息码
enum {
    TRANS_ID_PING_ABILITY = 5,
    TRANS_ID_REVERSED_MONITOR
};

const std::string DESCRIPTOR = "test.ITestAbility";

class ITestService : public IRemoteBroker {
public:
    // DECLARE_INTERFACE_DESCRIPTOR是必需的，入参需使用std::u16string；
    DECLARE_INTERFACE_DESCRIPTOR(to_utf16(DESCRIPTOR));
    virtual int TestPingAbility(const std::u16string &dummy) = 0; // 定义业务函数
};

class TestServiceProxy : public IRemoteProxy<ITestAbility> {
public:
    explicit TestAbilityProxy(const sptr<IRemoteObject> &impl);
    virtual int TestPingAbility(const std::u16string &dummy) override;
    int TestAnonymousStub();
private:
    static inline BrokerDelegator<TestAbilityProxy> delegator_; // 方便后续使用iface_cast宏
};

TestServiceProxy::TestServiceProxy(const sptr<IRemoteObject> &impl)
    : IRemoteProxy<ITestAbility>(impl)
{
}

int TestServiceProxy::TestPingAbility(const std::u16string &dummy){
    MessageOption option;
    MessageParcel dataParcel, replyParcel;
    dataParcel.WriteString16(dummy);
    int error = PeerHolder::Remote()->SendRequest(TRANS_ID_PING_ABILITY, dataParcel, replyParcel, option);
    int result = (error == ERR_NONE) ? replyParcel.ReadInt32() : -1;
    return result;
}
```




```c++
#include "iremote_object.h"

class TestDeathRecipient : public IRemoteObject::DeathRecipient {
public:
    virtual void OnRemoteDied(const wptr<IRemoteObject>& remoteObject);
}

void TestDeathRecipient::OnRemoteDied(const wptr<IRemoteObject>& remoteObject)
{
}
```

```c++
sptr<IPCObjectProxy> object = new IPCObjectProxy(1, to_utf16(DESCRIPTOR));
sptr<IRemoteObject::DeathRecipient> deathRecipient (new TestDeathRecipient());// 构造一个消亡通知对象
bool result = object->AddDeathRecipient(deathRecipient); // 注册消亡通知
result = object->RemoveDeathRecipient(deathRecipient); // 移除消亡通知
```

## JS侧接口

| 接口名               | 返回值类型 | 功能描述                                                     |
| -------------------- | ---------- | ------------------------------------------------------------ |
| addDeathRecippient   | boolean    | 注册用于接收远程对象消亡通知的回调，增加proxy对象上的消亡通知。 |
| removeDeathRecipient | boolean    | 注销用于接收远程对象消亡通知的回调。                         |
| onRemoteDied         | void       | 在成功添加死亡通知订阅后，当远端对象死亡时，将自动调用本方法。 |

### 参考代码

```ts
import FA from "@ohos.ability.featureAbility";
let proxy;
let connect = {
    onConnect: function(elementName, remoteProxy) {
        console.log("RpcClient: js onConnect called.");
        proxy = remoteProxy;
    },
    onDisconnect: function(elementName) {
        console.log("RpcClient: onDisconnect");
    },
    onFailed: function() {
        console.log("RpcClient: onFailed");
    }
};
let want = {
    "bundleName": "com.ohos.server",
    "abilityName": "com.ohos.server.EntryAbility",
};
FA.connectAbility(want, connect);
class MyDeathRecipient {
    onRemoteDied() {
        console.log("server died");
    }
}
let deathRecipient = new MyDeathRecipient();
proxy.addDeathRecippient(deathRecipient, 0);
proxy.removeDeathRecipient(deathRecipient, 0);
```

## Stub感知Proxy消亡（匿名Stub的使用）

正向的消亡通知是Proxy感知Stub的状态，若想达到反向的死消亡通知，即Stub感知Proxy的状态，可以巧妙的利用正向消亡通知。如两个进程A（原Stub所在进程）和B（原Proxy所在进程），进程B在获取到进程A的Proxy对象后，在B进程新建一个匿名Stub对象（匿名指未向SAMgr注册），可称之为回调Stub，再通过SendRequest接口将回调Stub传给进程A的原Stub。这样一来，进程A便获取到了进程B的回调Proxy。当进程B消亡或B所在设备离开组网时，回调Stub会消亡，回调Proxy会感知，进而通知给原Stub，便实现了反向消亡通知。

注意：

> 反向死亡通知仅限设备内跨进程通信使用，不可用于跨设备。

> 当匿名Stub对象没有被任何一个Proxy指向的时候，内核会自动回收。

### 参考代码

```c++
//Proxy
int TestAbilityProxy::TestAnonymousStub()
{
    MessageOption option;
    MessageParcel dataParcel, replyParcel;
    dataParcel.UpdateDataVersion(Remote());
    dataParcel.WriteRemoteObject(new TestAbilityStub());
    int error = Remote()->SendRequest(TRANS_ID_REVERSED_MONITOR,dataParcel, replyParcel, option);
    int result = (error == ERR_NONE) ? replyParcel.ReadInt32() : -1;
    return result;
}

//Stub

int TestAbilityStub::OnRemoteRequest(uint32_t code, MessageParcel &data, MessageParcel &reply, MessageOption &option)
{
    switch (code) {
        case TRANS_ID_REVERSED_MONITOR: {
            sptr<IRemoteObject> obj = data.ReadRemoteObject();
            if (obj == nullptr) {
                reply.WriteInt32(ERR_NULL_OBJECT);
                return ERR_NULL_OBJECT;
            }
            bool result = obj->AddDeathRecipient(new TestDeathRecipient());
            result ? reply.WriteInt32(ERR_NONE) : reply.WriteInt32(-1);
            break;
        }
        default:
            break;
    }
    return ERR_NONE;
}
```